1. Field of the Invention
The present invention relates to radio transceivers for direct sequence spread spectrum communications, and more particularly, to a wireless local area network in which each of a plurality of radio transceivers on the network can operate at different data rates simultaneously.
2. Description of Related Art
Spread spectrum modulation techniques are increasingly desirable for communications, navigation, radar and other applications. In a spread spectrum system, the transmitted signal is spread over a frequency band that is significantly wider than the minimum bandwidth required to transmit the information being sent. As a result of the signal spreading, spread spectrum systems have reduced susceptibility to interference or jamming, and enable high data integrity and security. Moreover, by spreading transmission power across a broad bandwidth, power levels at any given frequency within the bandwidth are significantly reduced, thereby reducing interference to other radio devices. In view of these significant advantages, spread spectrum communication systems are highly desirable for commercial data transmission.
In one type of spread spectrum communication system, a radio frequency ("RF") carrier is modulated by a digital code sequence having a bit rate, or chipping rate, much higher than a clock rate of the information signal. These spread spectrum systems are known as direct sequence ("DS"), or code division multiple access ("CDMA") modulation systems. The RF carrier may be binary or quadrature modulated by one or more data streams such that the data streams have one phase when a code sequence represents a data "one" and a predetermined phase shift (e.g., 180.degree. for binary, and 90.degree. for quadrature) when the code sequence represents a data "zero." These types of modulation are commonly referred to as binary phase shift key ("BPSK") and quadrature phase shift key ("QPSK") modulation, respectively.
It is also known to use a plurality of spread spectrum radio transmitter/receivers ("transceivers") that are coupled together in a wireless local area network ("WLAN"). A central host processing unit (i.e., a "network master" or "base station") sends information to and receives information from any one of the plurality of remotely disposed client transceiver nodes. In such a WLAN, the remote client transceivers may comprise portable units that operate within a defined environment to report information back to the network master. Each of the remote client transceivers communicate with the network master using the same RF carrier frequency and digital code sequence. It should be apparent that such WLAN systems offer increased flexibility over hard-wired systems by enabling operators of the remote transceivers substantial freedom of movement through the environment.
The individual client transceiver nodes amplify and filter an RF signal transmitted from the host processing unit to remove the RF carrier and provide a digital information signal that has been modulated by the digital code sequence. The client transceiver node then "de-spreads" the digital signal by use of a digital matched filter that is correlated with the digital code sequence to remove the modulation and recover the digital information. Discrete digital bits of the de-spread digital information are then assembled into packets having a predefined format that can be processed subsequently by use of conventional data processing logic systems, such as a microprocessor, digital signal processor, and the like.
In a communication system, energy gain can be defined as the signal-to-jamming ratio. The higher the signal-to-jamming ratio, the more immune the transceiver is to jamming interference (or background noise) which increases the effective range of the transceiver. For BPSK or QPSK communication systems operating without receiver diversity, the process gain is essentially identical to the energy gain. The process gain in spread spectrum processors may be defined as bandwidth (BW) available for communicating an information signal divided by the data rate (R.sub.b).
In decibels (dB), this ratio is defined as follows: EQU PG.sub.dB =10 log.sub.10 (BW/R.sub.b)
According to the preceding equation, for a fixed bandwidth, the processing gain increases as the data rate is decreased. By fixing the chipping rate, the bandwidth is also fixed. A fixed bandwidth is desirable since it allows optimization of the transmit and receive RF circuitry. Therefore, a lower data rate will provide more jamming or noise immunity than a higher data rate, and the processing gain of the transceiver is increased approximately 3 dB each time the data rate is halved. Even though the increased jamming immunity is desirable in such WLAN systems, the associated reduction in data rate tends to degrade the data throughput for the overall WLAN system.
In conventional WLAN system architectures, the Medium Access Control ("MAC") in the network master monitors the transceiver signal integrity. Each message transaction with a particular client transceiver node within the WLAN begins at a less complex modulation scheme, i.e., BPSK, and can be switched within the message transaction to a more complex modulation scheme, i.e., QPSK, so as to increase the data rate. If the received signal quality degrades below a set limit, the MAC will try to maintain a relatively reliable channel by switching back from QPSK to BPSK to reduce the data rate and increase the transceiver process gain. At the less complex modulation scheme of BPSK, the transceiver process gain is increased 3 dB, as described above. An advantage of this approach is that each client transceiver node can operate at a different data rate. Other WLAN system architectures that do not begin each message transaction at the less complex modulation scheme must reduce all transceivers to a lower data rate if signal integrity to any one transceiver becomes degraded. A drawback of each of these prior art WLAN system architectures is that the amount of transceiver process gain increase due to changing data rates is limited, and the MAC cannot readily optimize data rates with individual client transceiver nodes without repeated switching between data rates.
Thus, it would be desirable to provide a WLAN which can operate at multiple data rates simultaneously to optimize performance of each client transceiver node of the WLAN.